ESTRO 36 Abstract Book

S146 ESTRO 36 _______________________________________________________________________________________________

radiation oncologist and a resident radiation oncologist on all repeat CT scans and consensus was reached. The treatment plan of application one was projected on the repeat CT scans to simulate the other applications. Projected treatment plans were categorized as clinically acceptable or unacceptable. Additionally, new treatment plans were derived from the repeat CT scans by an experienced treatment planner. A conformity index, taking into account CTV coverage and dose to organs at risk, was used to quantify conformity of both the projected and the repeated treatment plans. Dose distributions were scaled to a prescription dose of 7 Gy. Using the Wilcoxon signed rank test, the conformity index and cumulative CTV D98 of the projected and repeated treatment plans were compared. Results Fourteen out of 22 projections were clinically unacceptable. In 8 of those 14 projections, replanning was of added value. In the remaining 6 unacceptable cases, replanning was of limited value as first an intervention would have been necessary to remove air and/or faeces. The figure shows a repeat CT with an unacceptable projection and corresponding replanning. The table summarizes the conformity index and cumulative CTV D98 of the non-adaptive and the adaptive approach. Parameters are presented both for all cases and for all cases excluding those that needed an intervention. Repeat CT-based adaptive HDR-BT resulted in a significantly higher conformity. Conclusion Repeat CT-based adaptive HDR-BT resulted in a more conformal treatment and should be standard practice in radical treatment with HDR-BT in rectal cancer patients.

on MRI are potentially smaller than CT-based volumes, which could lead to lower dose to organs at risk (OARs) and, in turn, reduction of RT-induced toxicity. The purpose of this study is to ascertain potential reduction in target volume and OAR dose. Material and Methods 23 breast cancer (cTis-3N0M0) patients from the MILANO trial (NL50046.041.14) were scanned in supine position on 1.5 T, arms abducted, after SN biopsy and breast- conserving surgery. MRI included a 3-dimensional (3D) T1- weighted (T1w) spoiled gradient echo (T1-SPGR) anatomical scan and two T2w fast spin echo (FSE) techniques for LN detection, which were co-registered. Axillary levels were delineated, using ESTRO guidelines [Offersen et al. 2015, IJROBP ], as well as OARs, including the lungs, heart, chest wall (CW), brachial plexus (BP), and humeral head (HH). LNs were identified by 4 observers, and delineated. Encompassing LN volumes – and after 5 mm isotropic expansion of the LNs – were related to axillary levels. In 5 patients (17–26 LNs), elective RT of 16 x 2.66 Gy = 42.56 Gy, delivered by 13 intensity- modulated RT beams, was simulated on MRI for two situations: (i) axillary levels I-IV, and (ii) all individual LN- based targets (1 mm PTV margin). For this, pseudo-CT scans were generated by bulk assignment of Hounsfield units on MRI for water, lungs and air. OAR dose parameters in both (i) and (ii) were compared. Results A median of 26 axillary LNs were delineated per patient. Compared to the respective axillary levels, LN-based target volumes, even after 5 mm isotropic expansion, are considerably smaller [table 1]. Coverage of all targets was excellent ( V 95% > 99%, V 107% = 0; all PTVs) in (i) and (ii). For elective RT on LN-based PTVs [figure 1], dose to all OARs was substantially reduced compared to standard elective RT: the average reduction of mean dose to lungs, heart, and HH was 2.3 Gy, 2.2 Gy, and 13.3 Gy, respectively; reduction of maximum dose to the BP and CW was 25.5 Gy and 9.4 Gy.

Poster Viewing : Session 6: Imaging

PV-0281 Lymph node MRI in regional breast radiotherapy leads to smaller target volumes and lower OAR dose T. Van Heijst 1 , H.J.G.D. Van den Bongard 1 , N. Hoekstra 1 , M.E.P. Philippens 1 , D. Eschbach 1 , J.J.W. Lagendijk 1 , B. Van Asselen 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective Elective axillary regional radiotherapy (RT) in breast cancer patients is performed with RT-planning CT scans, using delineation guidelines based on anatomical boundaries. In contrast to CT, MRI can directly image axillary lymph nodes (LNs) in RT position [van Heijst et al. 2016, BJR ]. Our MRI linac (MRL) system is designed to be able to treat those LNs precisely. LN-based target volumes